Systems and methods for remote monitoring of signals sensed by an implantable medical device during an MRI

ABSTRACT

Systems and methods are provided for allowing an implantable medical device, such as pacemaker, to properly sense electrophysiological signals and hemodynamic signals within a patient during a magnetic resonance imaging (MRI) procedure. Systems and methods are also provided for allowing the implantable medical device to transmit the sensed data to an external monitoring system during the MRI procedure so that attending medical personnel can closely monitor the health of the patient and the operation of the implantable device during the MRI. These improvements provide the attending personnel with information needed to determine whether the MRI should be suspended in response to induced tachyarrhythmias or other adverse conditions within the patient.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/938,088, filed Nov. 9, 2007, entitled “Systems and Methods for RemoteMonitoring of Signals Sensed by an Implantable Medical Device During anMRI,” now U.S. Pat. No. 8,200,334, which is related to U.S. patentapplication Ser. No. 10/973,862, filed Oct. 25, 2004,entitled“Electromagnetic Interference Safe Modes for ImplantableElectronic Devices”, now abandoned, which is fully incorporated byreference herein, including the appendices thereof.

FIELD OF THE INVENTION

The invention generally relates to implantable medical devices, such aspacemakers or implantable cardioverter/defibrillators (ICDs), and toexternal diagnostics systems for use therewith and, in particular, totechniques for monitoring and displaying electrophysiological signalsand hemodynamic signals sensed within a patient by an implantablemedical device during a magnetic resonance imaging (MRI) procedure.

BACKGROUND OF THE INVENTION

MRI is an effective, non-invasive magnetic imaging technique forgenerating sharp images of the internal anatomy of the human body, whichprovides an efficient means for diagnosing disorders such asneurological and cardiac abnormalities and for spotting tumors and thelike. Briefly, the patient is placed within the center of a largesuperconducting magnetic that generates a powerful static magneticfield. The static magnetic field causes protons within tissues of thebody to align with an axis of the static field. A pulsed radio-frequency(RF) magnetic field is then applied causing the protons to begin toprecess around the axis of the static field. Pulsed gradient magneticfields are then applied to cause the protons within selected locationsof the body to emit RF signals, which are detected by sensors of the MRIsystem. Based on the RF signals emitted by the protons, the MRI systemthen generates a precise image of the selected locations of the body,typically image slices of organs of interest.

A significant problem with MRI is that its strong magnetic fields caninterfere with the operation of any medical devices, particularlypacemakers or ICDs, implanted within the patient. Typically, pacemakersand ICDs include pulse generators for generating electrical pacingpulses and shocking circuits for generating stronger defibrillationshocks. A set of conductive leads connect the pulse generators andshocking circuits to electrodes implanted within the heart. Anindividual pacing pulse is applied by using the pulse generators togenerate a voltage difference between a pair of the electrodes, such asbetween a tip electrode implanted within the right ventricle and thepacemaker housing or “can.” A defibrillation shock is applied by usingthe shocking circuits to generate a much larger voltage differencebetween a pair of the electrodes, such as between a large coil electrodeimplanted within the right ventricle and the pacemaker housing. Theleads may also have a variety of sensors for sensing physiologicalsignals within the heart of the patient, such as pressure sensor,temperature sensors, SvO₂ sensors, PPG and the like. The sensors aretypically connected to the implantable device via electrical signalconduction paths within the various leads so as to receive controlsignals from the implanted device and to relay sensed signals back tothe device. The pulse generators, shocking circuits, leads, electrodesand sensors, as well as the tissue and fluids between the electrodes andsensors, collectively provide various conduction loops. State of the artpacemakers and ICDs exploit lead systems having numerous electrodes andsensors, thus presenting numerous possible conduction paths.

When patients with pacers or ICDs are exposed to MRI fields, RF fieldsof the MRI can induce currents along the conduction paths causingexcessive current to flow into tissue or blood through tip or ringelectrodes resulting in Joule heating. Excessive power dissipation mightdamage the tissues around pacing electrodes causing inappropriatesensing and pacing and posing risks to the patient. Additionally, thepowerful gradient fields of an MRI system can induce currents among theconduction paths sufficient to trigger rapid, unwanted pacing pulses oreven defibrillation shocks. These induced currents are referred to asparasitic currents. Rapid pacing pulses induced by the MRI could, incertain cases, cause a life-threatening fibrillation of the heart.Likewise, any defibrillation shocks triggered by the presence of the MRIfields can also induce fibrillation, particularly if the shock isdelivered during a repolarization period of the ventricular myocardium.Another significant concern is that the induced voltages can bemistakenly sensed by the pacemaker as intrinsic heartbeats. In somepacing modes, particularly demand-based modes, the pacemaker thenassumes that the heart needs no pacing assistance and will block itspacing output (i.e. delivery of a pacing pulse is inhibited.) This couldcause a “pacing dependent” patient to pass out and possibly die.

In view of these concerns, various safeguard techniques have beendeveloped that operate to detect the strong fields associated with anMRI and then switch sensing modes or pacing modes in response thereto.See, for example, U.S. Patent Application 2003/0083570 to Cho et al.;U.S. Patent Application 2003/0144704 to Terry et al.; U.S. PatentApplication 2003/0144705 to Funke; U.S. Patent Application 2003/0144706also to Funke; U.S. Pat. No. 6,795,730 to Connelly et al., and U.S.Patent Application 2004/0088012 of Kroll et al.

However, it would be preferable to allow the implanted device tocontinue to operate in its normal pacing modes even during an MRIprocedure, so long as heating criteria is met, arrhythmias are notinduced, unnecessary pacing pulses or shocks are not delivered, and anynecessary therapy is not improperly inhibited. That is, it would bedesirable to allow the device to continue to monitor the heart of thepatient for arrhythmias or other medical conditions even during an MRIprocedure and to deliver therapy as needed and to transmit signals todeactivate the MRI system only if absolutely necessary. With such asystem, it would also be desirable to control the device to transmitmonitoring and diagnostic information during the MRI procedure to anexternal monitoring and control system so that medical personnel canmonitor the status the implanted device and the health of the patientduring the MRI procedure. The medical personnel then could deactivatethe MRI system if warranted or adjust its operation if needed. Themedical personnel could also re-program the operation of the implanteddevice during the MRI procedure, if appropriate. The implanted devicewould preferably also monitor for any arrhythmias or other abnormalmedical conditions induced by the MRI fields and send appropriatesignals to the MRI system to automatically deactivate the MRI system. Inparticular, the device would monitor for any tachyarrhythmias induced bythe MRI fields so that the MRI system can be promptly deactivated andappropriate therapy delivered.

With conventional implantable systems, though, the strong MRI fields canprevent the implanted device from reliably sensing signals from thevarious electrodes of the leads and from the various physiologicalsensors, thus preventing the implanted device from reliably detectingarrhythmias or other abnormal conditions within the patient during theMRI procedure. Accordingly, there is a need to provide improvedimplantable components configured to allow the implanted device tocontinue to reliably receive signals from sensing leads andphysiological sensors during an MRI and it is to this end that certainaspects of the invention are directed. Moreover, the strong MRI fieldscan also prevent the implanted device from reliably sendingtransmissions to, and receiving signals from, an external system, thuspreventing the implanted device from reliably sending diagnostics dataand warning signals to the external system during the MRI procedure andpossibly also preventing the external system from transmittingre-programming commands to the implanted device during the MRIprocedure. Accordingly, there is also a need to provide improvedimplantable components and external components sufficient to allow theimplanted device and the external system to reliably communicate withone another during an MRI procedure and it is to this end that otheraspects of the invention are directed.

Still another significant concern is that the MRI fields can cause tipelectrodes of the leads to become significantly heated, potentiallydamaging adjacent tissues. Techniques have been developed for detectingthe heating of tip electrodes and deactivating the MRI system inresponse thereto. See, for example, U.S. Patent Application2006/0025820, of Phillips et al., entitled “Integrated System and Methodfor MRI-safe Implantable Devices.” Typically, though, the implanteddevice merely determines whether the tip temperature has exceeded athreshold and sends signals to deactivate the MRI system. It would bepreferable to additionally track changes in tip temperatures so as toprovide other diagnostic information and, in particular, to exploitchanges in tip temperature to determine the amount of current induced ina given lead by the MRI fields. It is to this end that still otheraspects of the invention are directed.

SUMMARY

In accordance with a first general embodiment, methods are provided foruse by an implantable medical device for implant within a patient,wherein the method is for use during an MRI procedure. In one example,signals are sensed within the patient during the MRI procedure whilefiltering the sensed signals to reduce the influence of MRI fields onthe sensed signals and then the signals are transmitted to an externalmonitoring system during the MRI procedure so that the signals canthereby be remotely monitored during the MRI. The signals sensed by theimplanted device and then transmitted to the external system includeelectrophysiological signals such as intracardiac electrogram (IEGM)signals. The signals sensed by the device and then transmitted to theexternal system can also include hemodynamic signals such as:intracardiac pressure signals, blood oxygen saturation signals, bloodtemperature signals, and photoplethysmography (PPG) signals. Devicediagnostic signals can also be transmitted to the external system, suchas signals representative of tip temperatures or induced currents. Byproviding all or at least some of these signals to the externalmonitoring system during the MRI, medical personnel can thereby closelymonitor the health of the patient and the operation of the implantabledevice during the MRI, hence providing the attending medical personnelwith information needed to determine whether the MRI should be suspendedin response to induced tachyarrhythmias or other adverse conditionswithin the patient.

In an exemplary embodiment, the electrophysiological and hemodynamicsignals sensed by the implantable device are filtered using filtersconfigured to reduce the influence of MRI fields on the sensed signals,such as by filtering the signals at frequencies associated with MRIfields. For example, 64 megahertz (MHz) and 128 MHz notch filters may beprovided within terminals of the implanted device that receive signalsfrom sensing/pacing leads and within terminals that receive signals fromphysiological sensors. The notch filters substantially eliminate anysignal components arising at the MRI frequencies from entering thesensing circuitry within the implanted device, thus allowing the deviceto more reliably sense electrophysiological and hemodynamic signals evenwhile MRI fields are being applied. Additionally, or alternatively,adaptive slew rate filters are provided for filtering signals at slewrates associated with MRIs, such as slew rates in the range of 100milliTesla per meter per millisecond (mT/m/ms) to about 400 mT/m/ms. Insome embodiments, it may be appropriate to also provide notch filtersand/or slew rate filters within the leads or sensors themselves,particularly within any physiological sensors that receive command andcontrol signals from the implantable device.

In the exemplary embodiment, any signals to be transmitted to theexternal monitoring system during the MRI procedure are transmitted atfrequencies associated with medical implant communication services(MICS) band frequencies, particularly frequencies in the range of 402MHz-405 MHz. Alternatively, the signals are transmitted at frequenciesassociated with industrial scientific medical (ISM) band frequencies,particularly frequencies in the range of about 2.5 GHz-5.0 GHz. Bytransmitting signals at MICS band or ISM band frequencies, theimplantable device can more reliably transmit the electrophysiologicaland hemodynamic signals, and any other diagnostic signals, to theexternal monitoring system during the MRI procedure for display. Medicalpersonnel can then review the information received from the implantabledevice to verify that the implantable device is operating safely withinthe patient and to deactivate the MRI system if necessary or tore-program the operation of the implantable device, if warranted. Inimplementations where the external system is equipped to re-program orotherwise adjust the operation of the implantable device during the MRIprocedure, MICs band or ISM band frequencies can also be used totransmit the appropriate programming signals to the device under thecontrol of the attending medical personnel.

In addition to relaying electrophysiological and hemodynamic signals tothe external monitoring system, the implantable device also preferablyanalyzes the sensed signals to detect any abnormal conditions within thepatient such as arrhythmias, abnormal patient blood temperatures andabnormal patient blood pressures. Upon detection of any abnormalconditions, appropriate warning signals are sent to the externalmonitoring system for review by the medical personnel. Warningspertaining to tachyarrhythmias are responded to immediately by attendingpersonnel by, e.g., removing the patient from the MRI and promptlydelivering any needed therapy.

Depending upon the nature of the arrhythmia, the device may alsoautomatically change its pacing mode in response to the arrhythmia.Likewise, depending upon the nature of the arrhythmia, the externalmonitoring system, upon reception of the warning signals, mayautomatically deactivate the MRI system. Still further, the externalmonitoring system may analyze the various electrophysiological andhemodynamic signals received from the implantable device to detect anyabnormal conditions that the device may not have detected and togenerate warning signals accordingly. In this regard, more sophisticatedanalysis procedures may be employed by the external system to detectarrhythmias, i.e. analysis procedures that the implantable device maynot have the resources to perform. Should an arrhythmia be detected, theMRI is deactivated and appropriate therapy is immediately delivered bythe attending personnel.

Preferably, temperature sensors are provided within the various leadsfrom which the implantable device can detect any abnormally high tiptemperatures so that appropriate warning signals can be sent to theexternal monitoring system or to the MRI system itself. Still further,the implantable device preferably tracks temperature profiles for one ormore of the tip electrodes, i.e. changes in tip temperature over timeduring MRI scans, from which either the device or the externalmonitoring system can determine the amount of current induced within theleads. Appropriate warning signals can be generated or other actionstaken in response to any induced currents that exceed a predeterminedacceptable threshold.

Preferably, the aforementioned procedures performed by the implantabledevice are initiated prior to the initiation of the MRI procedure so asto provide medical personnel with “baseline” electrophysiological andhemodynamic signals obtained within the patient before the MRI procedurefor comparison against subsequent signals obtained during the MRI. Inone example, the external monitoring system transmits a suitable MRIsystem notification signal, which the implantable device is equipped todetect. In this manner, the implantable device can detect entry into theMRI procedure room and promptly initiate an MRI operational mode wherethe aforementioned signals filtering and transmission procedures areperformed. Alternatively, the external monitoring system or an externalprogrammer device may be used by medical personnel within the MRIprocedure room to manually re-program the implantable device to switchto the MRI operational mode.

Although thus far summarized primarily with reference to the operationsperformed by the implantable medical device during an MRI procedure, itshould be understood that aspects of the invention may be implementedpartially or exclusively within external systems. That is, in accordancewith a second general embodiment of the invention, methods are providedfor use by an external monitoring system used in conjunction with animplantable medical device for implant within a patient. Briefly,signals are received by the external monitoring system via long-rangetelemetry during a magnetic imaging procedure that had been transmittedfrom an implantable medical device implanted within a patient undergoingthe procedure. The signals, which may include electrophysiological andhemodynamic signals, are received at frequencies selected to reduce theinfluence of magnetic imaging fields on the transmitted signals. Theexternal monitoring system processes the received signals during theimaging procedure, such that signals sensed within the patient duringthe procedure can be remotely monitored during the procedure. As alreadyexplained, such processing can include displaying the received signals,analyzing the signals to detect abnormal conditions within the patient,etc. Additionally, tip temperature signals, induced current signals,device diagnostics, and various warnings can be received and displayedby the external monitoring system. Warnings pertaining totachyarrhythmias are responded to immediately by attending personnel by,e.g., removing the patient from the MRI and promptly delivering anyneeded therapy.

Any or all of the various electrophysiological and hemodynamic signals,tip temperature signals, induced current signals, device diagnostics,warnings and the like can be forwarded from the external monitoringsystem, which is preferably mounted near the MRI system, to a remotemonitoring terminal for review by other medical personnel. For example,the various signals and warnings can be selectively forwarded via theInternet to a cardiologist or electrophysiologist at a remote location.In particular, if any abnormal conditions arise within the patient thatthe personnel operating the MRI system are unable to diagnose,appropriate diagnostic data can thereby be promptly forwarded to anexpert who can then recommend appropriate action. However, theforwarding of data for remote review is not performed in lieu ofremoving a patient from the MRI in response to a tachyarrhythmia.Rather, any serious arrhythmia is addressed immediately by the MRIpersonnel. Nevertheless, circumstances may arise where it is desirableto have IEGMs or other diagnostic data reviewed by experts at a remotelocation, time permitting.

The invention is perhaps most advantageously implemented for use withMRI systems but principles of the invention may be exploited for usewith other systems providing strong magnetic fields as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features, advantages and benefits of the inventionwill be apparent upon consideration of the descriptions herein taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a stylized representation of an MRI system along with apatient with a pacer/ICD implanted therein that is capable ofcommunicating with an external monitoring system during an MRIprocedure;

FIG. 2 is a flow diagram providing a broad overview of techniquesperformed by the pacer/ICD and the external monitoring system of FIG. 1;

FIG. 3 is a flow diagram providing a more detailed illustration ofexemplary processing techniques in accordance with the generaltechniques of FIG. 2, particularly signal sensing and filteringprocedures performed by the pacer/ICD during an MRI procedure;

FIG. 4 is a graph illustrating the efficacy of filtering techniquesperformed by the pacer/ICD of FIG. 1 during an MRI procedure;

FIG. 5 is a flow diagram providing a more detailed illustration ofexemplary processing techniques performed in accordance with the generaltechniques of FIG. 2, particularly signal transmission proceduresperformed by the pacer/ICD during the MRI procedure;

FIG. 6 is a flow diagram providing a more detailed illustration ofexemplary processing techniques performed in accordance with the generaltechniques of FIG. 2, particularly signal reception procedures performedby the external monitoring system during the MRI procedure;

FIG. 7 is a flow diagram providing a more detailed illustration ofexemplary processing techniques performed in accordance with the generaltechniques of FIG. 2, particularly signal analysis and displayprocedures performed by the external monitoring system during the MRIprocedure;

FIG. 8 is a simplified, partly cutaway view, illustrating the pacer/ICDof FIG. 1 along with a full set of leads implanted in the heart of thepatient;

FIG. 9 is a functional block diagram of the pacer/ICD of FIG. 8,illustrating basic circuit elements that provide cardioversion,defibrillation and/or pacing stimulation in four chambers of the heartand particularly illustrating components for MRI-mode processing; and

FIG. 10 is a functional block diagram illustrating components of anexternal monitoring system for use in receiving, analyzing anddisplaying signals received from the pacer/ICD of FIG. 9 during an MRIprocedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode presently contemplatedfor practicing the invention. The description is not to be taken in alimiting sense but is made merely to describe general principles of theinvention. The scope of the invention should be ascertained withreference to the issued claims. In the description of the invention thatfollows, like numerals or reference designators will be used to refer tolike parts or elements throughout.

Overview of MRI-Responsive Systems and Procedures

FIG. 1 illustrates an overall MRI system 2 having an MRI machine 4operative to generate MRI fields during an MRI procedure for examining apatient. The MRI machine operates under the control of an MRI controller6, which controls the strength and orientation of the fields generatedby the MRI machine and derives images of portions of the patienttherefrom, in accordance with otherwise conventional techniques. MRImachines and imaging techniques are well known and will not be describedin detail herein. See, for example, U.S. Pat. No. 5,063,348 to Kuhara etal., entitled “Magnetic Resonance Imaging System” and U.S. Pat. No.4,746,864 to Satoh, entitled “Magnetic Resonance Imaging System.” Anexternal monitoring system 8 is also provided that communicates via longrange RF telemetry during the MRI procedure with a pacer/ICD 10implanted within the patient to receive transmissions ofelectrophysiological signals and/or hemodynamic signals sensed withinthe patient by the pacer/ICD during the MRI procedure, as well as otherdiagnostic data to be described in greater detail below. A lead system12 is coupled to the pacer/ICD for sensing electrophysiological signalswithin the heart of the patient, such as A-IEGM and V-IEGM signals, andfor delivering any needed pacing pulses or shock therapy. In FIG. 1,only two leads are shown. A more complete lead system is illustrated inFIG. 9, described below. In general, any of the electrophysiologicalsignals sensed using the pacing/sensing leads might potentially betransmitted to the external monitoring system during the MRI procedurefor display thereon.

The lead system may also include various physiological sensors (notseparately shown within FIG. 1) for sensing hemodynamic signals or othersignals within the patient, such as sensors operative to senseintracardiac pressure, blood oxygen saturation (i.e. blood SO₂), bloodtemperature, and PPG signals, etc. In some cases, the sensors may beimplanted elsewhere in the patient or may be mounted in or on thepacer/ICD itself. In any case, any of the various hemodynamic signals orother signals sensed using the sensors might potentially be transmittedto the external monitoring system during the MRI procedure for displaythereon. Also, as will be further explained, during the MRI procedure,the pacer/ICD also analyzes the various sensed signals to detectabnormal conditions such as tachyarrhythmias, sudden drops in bloodpressure, sudden changes in blood temperature, etc. Warning signalspertaining to any such abnormal conditions may also be transmitted fromthe pacer/ICD to the external monitoring system during the MRI procedurefor review. Still further, the lead system is preferably equipped withone or more temperature sensors (not shown in FIG. 1) for detecting tiptemperatures from which the pacer/ICD estimates the amount of electricalcurrent, if any, induced within the leads by the MRI fields. Warningsand other information pertaining to tip temperatures or induced currentsmay also be transmitted to the external monitoring system during the MRIprocedure for review. Depending upon the nature of the warning, theexternal monitoring system may deactivate the MRI machine by sendingappropriate control signals to the MRI controller. Alternatively,medical personnel operating the system may manually deactivate the MRImachine in response to the warnings by using an MRI control interface,not separately shown.

External monitoring system 8 is also preferably equipped to analyze anyof the various signals received from the pacer/ICD to detect abnormalconditions, including abnormal tip temperatures, abnormal inducedcurrent levels, tachyarrhythmias, etc., and to generate suitable warningsignals for the attending personnel and to directly deactivate the MRI,if appropriate. In particular, the external monitoring system may beprovided with more sophisticated software or hardware than includedwithin the pacer/ICD itself, for use in analyzing IEGMs, cardiacpressure signals, and the like to detect abnormal conditions. In thisregard, the external monitoring system may be provided with softwarerequiring greater memory or processing resources than available withinthe pacer/ICD. Where appropriate, information received or generated bythe external monitoring system is forwarded via the Internet or otherappropriate communications network to a remote monitoring terminal 14for review thereon. In particular, IEGMs corresponding to any abnormalcardiac conditions that neither the external monitoring system nor theattending personnel are able to diagnose may be forwarded to the remotemonitor for review by an electrophysiologist or cardiologist withgreater expertise.

To permit communication with the pacer/ICD during the MRI procedure, theexternal monitoring system includes an RF telemetry antenna 16 thatcommunicates via MICS or ISM channels with corresponding RF telemetrycomponents within the pacer/ICD (shown in FIG. 9, discussed below).Preferably, the RF telemetry antenna of the external monitoring systemalso periodically emits suitable MRI notification signals at all times.The pacer/ICD is equipped to sense such notification signals to therebydetect entry of the patient into the MRI procedure room. Upon detectionof such entry, the pacer/ICD switches to an MRI mode wherein thepacer/ICD activates various filtering procedures (to be discussed below)for use in the presence of MRI fields and also promptly beginstransmitting IEGMs and other signals of interest via MICS or ISMfrequencies to the external monitoring system for display thereon, so asto provide baseline signals for comparison against signals subsequentlytransmitted during the MRI procedure. The pacer/ICD continues to operatein the MRI mode throughout the MRI procedure and does not switch back tonormal processing modes until the patient has eventually been removedfrom the MRI procedure room, as detected based on loss of reception ofthe periodic notification signals.

FIG. 2 broadly summarizes the operations performed by the pacer/ICD andthe external monitoring system of FIG. 1 while in the MRI mode. Briefly,beginning at step 100, the pacer/ICD senses electrophysiologicalsignals, hemodynamic signals and/or other signals within the patientduring the MRI procedure, while filtering the sensed signals to reducethe influence of MRI fields on the sensed signals. At step 102, thepacer/ICD transmits the sensed signals to the external monitoring systemduring the MRI procedure at frequencies selected to reduce the influenceof the MRI fields on the transmitted signals so that the patient can beremotely monitored during the MRI procedure. At step 104, theelectrophysiological signals, hemodynamic signals and/or other signalstransmitted by the pacer/ICD during the MRI procedure are received bythe external monitoring system at the frequencies selected to reduce theinfluence of the MRI fields on the signals and/or the interaction of thesignals with the fields. At step 106, the external monitoring systemprocesses and displays the received signals during the MRI procedure,such that signals sensed within the patient can be monitored during theMRI procedure by attending personnel.

Illustrative MRI-Responsive Systems and Procedures

FIGS. 3-7 set forth illustrative embodiments of the invention.Operations performed by the pacer/ICD are set forth in FIGS. 3-5;operations performed by the external monitoring system or other externalsystems are set forth in FIGS. 6-7.

FIG. 3 illustrates exemplary techniques that may be performed by thepacer/ICD in accordance with step 100 of FIG. 2. Beginning at step 200,the pacer/ICD (1) detects the proximity to an MRI system via long-rangeRF telemetry, (2) receives short-range notification signals from adevice programmer notifying it of the MRI, or (3) directly detects thepresence of MRI fields. As noted, the external monitoring systempreferably transmits notification signals periodically via MICs band orISM band long-range telemetry, which the pacer/ICD is equipped to sense,so as to detect entry of the patient into an MRI procedure room. MICSband frequencies are in the range of 402 MHz-405 MHz. ISM bandfrequencies are in the range of 2.5 GHz-5.0 GHz. Upon detection of theMRI notification signals, the pacer/ICD automatically switches to an MRImode of operation where, as will be explained, it begins to sensesignals subject to special MRI filtering procedures, analyzes thesignals accordingly, and begins transmitting the signals to the externalmonitoring system for review, etc. If the external monitoring system isnot equipped to transmit such notification signals, or if the pacer/ICDis not equipped to receive and respond to those notification signals,the pacer/ICD can nevertheless be switched to the MRI mode of operationbased on short-range telemetry signals received from a deviceprogrammer. That is, the pacer/ICD of the patient may be re-programmedby a standard device programmer, using otherwise conventionalprogramming techniques, to switch to the MRI mode before the patient isplaced in the MRI machine. (Note that the device programmer may also beequipped to perform all the functions of the external monitoring system,so that separate device programmers and external monitoring systems arenot necessarily required.) If the pacer/ICD is not, for whatever reason,switched to the MRI mode prior to initiation of the MRI procedure, thepacer/ICD nevertheless detects the presence of the MRI fields using,e.g., suitable magnetic field sensors installed within the pacer/ICD,and automatically switches to the MRI mode.

So long as no MRI notification signals, programming signals, or MRIfields are sensed, normal processing is performed by the pacer/ICD, atstep 202. By “normal” processing, it is meant that the pacer/ICDperforms functions in a manner that does not specifically take intoaccount the presence of strong magnetic fields. For a pacer/ICD, normalfunctions involve any of a variety of cardiac rhythm managementfunctions, such as anti-bradycardia pacing, anti-tachycardia pacing(ATP), overdrive pacing, and the like, that involve deliveringelectrical stimulation to heart tissue using otherwise conventionalsensing and analysis techniques. For other implantable medical devices,such as neural stimulators or the like, normal processing may involvethe delivery of electrical stimulation to nerves or other tissues, againin a manner that that does not specifically take into account thepresence of strong magnetic fields.

Assuming, however, that the pacer/ICD is switched to the MRI mode, thenstep 204 is performed wherein the pacer/ICD activates an MRI sensingmode where MRI filters are employed along sensing channels, including 64MHz and 128 MHz notch filters and/or MRI slew rate filters operating inthe range of 100 mT/m/ms to about 400 mT/m/ms with, preferably, adaptivefiltering. As far as the notch filters are concerned, such filters maybe permanently mounted within feedthrough filters provided at each inputterminal, preferably including each pacing/sensing lead feedthrough andeach physiological sensor lead feedthrough. (Feedthrough filters arediscussed in, e.g., U.S. patent application Ser. No. 11/256,480,Unpublished, filed Oct. 20, 2005, of Propato et al., entitled “ImprovedFeedthrough Filter for Use in an Implantable Medical Device,” nowabandoned. See, also, U.S. patent application Ser. No. 11/450,945,Unpublished, filed Jun. 9, 2006, of Propato, entitled “MultilayerL-section Filter for use in an Implantable Medical Device,” nowabandoned.) That is, the notch filters continuously filter inputsignals, whether in the MRI mode or not. However, in otherimplementations, the notch filters may be configured so as to beactivated or inserted along the signals lines only during the MRI mode.Notch filters may also be mounted at the input terminals of anyphysiological sensors that receive command or control signals from thepacer/ICD, particularly any sensors that preferably operate even duringan MRI, such as cardiac pressure sensors. In any case, the various notchfilters substantially eliminate signals at 64 MHz and 128 MHz, which arethe typical operating frequencies of MRI systems, and hence the filterssubstantially eliminate any components of sensed signals that might becaused by the MRI, rather than by the electrophysiological and/orhemodynamic phenomena of interest. In particular, by filtering out MRIsignals from electrophysiological and/or hemodynamic signals, thepacer/ICD will not erroneously respond to the MRI signals and willrespond instead only to the underlying signal of interest.

Insofar as the slew rate and/or adaptive filters are concerned, suchfilters are preferably activated only within the MRI mode. Again, suchfilters may be provided at all input signal feedthroughs, both withinthe pacer/ICD and within any physiologic sensors, particularly thosethat are intended to operate even during an MRI. Slew rate filters arediscussed in, e.g., U.S. Pat. No. 6,052,614 to Morris et al.,“Electrocardiograph Sensor and Sensor Control System for Use withMagnetic Resonance Imaging Machines.” See, also, U.S. Pat. No. 7,039,455to Brosovich et al., “Apparatus and Method for Removing MagneticResonance Imaging-Induced Noise from ECG Signals.” Adaptive filteringtechniques can be additionally or alternatively be applied. See, forexample, U.S. Pat. No. 6,675,036 to Kreger et al., entitled “DiagnosticDevice Including a Method and Apparatus for Bio-Potential NoiseCancellation Utilizing the Patient's Respiratory Signal.” Theaforementioned patents generally pertain to the filtering of ECG signalsbut such filtering techniques can be applied, where appropriate, to thefiltering of IEGM signals, physiological signals, etc.

FIG. 4 illustrates the effectiveness of slew rate filters and adaptivefilters. A first graph 206 illustrates an IEGM in the presence of MRInoise. A second graph 208 illustrates the same IEGM, filtered using aslew rate filter set based on the characteristics of the MRI. As can beseen, the IEGM is more clearly presented. A third graph 210 illustratesthe same IEGM, additionally filtered using an adaptive filter. The IEGMis even more clearly presented.

Returning to FIG. 3, at step 212, the pacer/ICD senses one or more of:IEGM signals, such as separate A-IEGM and V-IEGM channels; intracardiacpressure signals, such as left atrial pressure (LAP), right ventricularpressure (RVP); blood oxygen saturation signals, such as separate venousoxygen saturation (SvO₂) and arterial oxygen saturation (SaO₂) signals;patient temperature values, such as blood temperature or bodytemperature; PPG signals; and tip temperature values. The varioussignals or values are sensed using the filtered sensing techniquesdiscussed in step 204. As such, any noise with the sensed parameters dueto the MRI fields is substantially reduce or eliminated. These sensedparameters are merely exemplary. A wide variety of otherelectrophysiological and/or hemodynamic parameters can additionally, oralternatively, be sensed. Examples include, pH values, blood glucosevalues, accelerometer values, stroke volume, cardiac output values,contractility values, respiration, acoustic sensor values and enddiastolic volume (EDV) values.

The sensors themselves can be otherwise conventional. However,particularly effective techniques for detecting blood pressure valuesare discussed in U.S. patent application Ser. No. 11/378,604, filed Mar.16, 2006, of Kroll et al., entitled, “System and Method for DetectingArterial Blood Pressure based on Aortic Electrical Resistance using anImplantable Medical Device,” now U.S. Pat. No. 7,654,964. Particularlyeffective techniques for detecting blood oxygen saturation values arediscussed in U.S. patent application Ser. No. 11/387,579, filed Mar. 23,2006, of Koh, entitled “System and Method for Calibrating a Blood OxygenSaturation Sensor for use with an Implantable Medical Device,” now U.S.Pat. No. 8,099,146. Particularly effective techniques for detectingstroke volume and/or cardiac output values are discussed in U.S. patentapplication Ser. No. 11/267,665, filed Nov. 4, 2005, of Kil et al.,entitled “System and Method for Measuring Cardiac Output via ThermalDilution using an Implantable Medical Device with Thermistor Implantedin Right Ventricle,” now abandoned. Particularly effective techniquesfor detecting EDV values are discussed in U.S. Patent Publication No.2005/0215914, to Bornzin et al., entitled “System and Method forEvaluating Heart Failure Based on Ventricular End-Diastolic Volume usingan Implantable Medical Device.” Particularly effective techniques fordetecting contractility values are described in: U.S. Pat. No. 5,800,467to Park et al., entitled “Cardio-Synchronous Impedance MeasurementSystem for an Implantable Stimulation Device.” Particularly effectivetechniques for detecting respiration are described in: U.S. patentapplication Ser. No. 11/100,189, filed Apr. 5, 2005, of Koh, entitled“System and Method for Detection of Respiration Patterns via Integrationof Intracardiac Electrogram Signals,” now U.S. Pat. No. 7,404,799.Particularly effective techniques for mounting multiple sensors toindividual leads are described in U.S. patent application Ser. No.11/623,663, filed Jan. 16, 2007, of Zou et al., entitled “Sensor/LeadSystems for use with Implantable Medical Devices,” now U.S. Pat. No.8,388,670. Particularly effective techniques for detecting tiptemperature include placing temperature sensors on leads by winding thesensor wire so as to cancel induced current or by placing the wiresinside suitable coaxial outer conductors or by using fiber optictechniques.

At step 214, the pacer/ICD then analyzes the tip temperature signals toestimate the amount of current induced in the leads. That is, for eachtip electrode for which a temperature profile can be ascertained basedon a series of tip temperature values detected over a period of time,the pacer/ICD estimates the current induced along the lead in which thatparticular tip electrode is mounted. If tip temperature values areobtained for each lead, then the pacer/ICD can estimated the currentinduced in each lead. In this regard, RF heating of tip electrode isclosely related to the specific absorption rate (SAR) of the tipelectrode material and the induced current through the lead. (SAR isdefined as the RF power absorbed per unit of mass of an object, and ismeasured in watts per kilogram (W/kg).) Hence, induced current levelscan be estimated based on tip temperatures. For example, therelationship between tip temperature and induced current for aparticular lead may be established in advance via otherwise conventionaltechniques, e.g. linear regression, with the numerical relationship thenprogrammed via software within the pacer/ICD. Thereafter, changes in tiptemperature can be converted into induced current estimates by thepacer/ICD.

At step 216, the pacer/ICD analyze any or all of the various signalsthat have been sensed and any values derived therefrom to detect anyabnormal conditions within the patient including: arrhythmias, such asventricular tachyarrhythmias induced by the MRI; abnormal patienttemperatures, such as abnormal blood temperatures or body temperatures;abnormal cardiac pressure values, such as sudden drops in bloodpressure; abnormal tip temperatures; and/or abnormal induced currentlevels. In this regard, various threshold values indicative of “normal”ranges of temperature and pressure values may be preprogrammed withinthe pacer/ICD for comparison purposes to detect abnormal conditions.Otherwise conventional techniques for distinguishing among differenttypes of arrhythmias may be employed. For example, a ventricular ratesensed within the patient can be compared against separate VT and VFthresholds. If the rate exceeds a higher VF threshold (set, e.g., to 220beats per minute (bpm)), then VF is detected. If the rate only exceedsthe lower VT threshold (set, e.g., to 180 bpm), then VT is detected.

At step 217, the pacer/ICD delivers suitable therapy in response to theabnormal condition. Also, in response to a ventricular tachyarrhythmia,the pacer/ICD may perform an Automatic Mode Switch (AMS), wherein thepacemaker reverts from a tracking mode such as a VDD or DDD mode to anontracking mode such as WI or DDI mode. WI and DDI are standard devicecodes that identify the mode of operation of the device. Others standardmodes include DDD, VDD and VOO. Briefly, DDD indicates a device thatsenses and paces in both the atria and the ventricles and is capable ofboth triggering and inhibiting functions based upon events sensed in theatria and the ventricles. VDD indicates a device that sensed in bothchambers but only paces in the ventricle. A sensed event on the atrialchannel triggers a ventricular output after a programmable delay. WIindicates that the device is capable of pacing and sensing only in theventricles and is only capable of inhibiting the functions based uponevents sensed in the ventricles. DDI is identical to DDD except that thedevice is only capable of inhibiting functions based upon sensed events,rather than triggering functions. As such, the DDI mode is anon-tracking mode precluding its triggering ventricular outputs inresponse to sensed atrial events. VOO identifies fixed-rate ventricularpacing, which ignores any potentially sensed cardiac signals. This modeis quite different from the aforementioned “demand” modes, which onlypace when the pacemaker determines that the heart is “demanding” pacing.Numerous other device modes of operation are possible, each representedby standard abbreviations of this type. In some implementations, at step217, the pacer/ICD might also deliver certain kinds of therapy inresponse to an arrhythmia. Preferably, though, any such therapy issuspended pending deactivation of the MRI machine, as the heart mightrevert to a normal sinus rhythm once the MRI is deactivated. Also,attempts to deliver therapy during an MRI might cause damage. Forexample, strong currents from cardioversion or defibrillation shocksmight generate high force/torque that could damage the leads and causesevere heating. Accordingly, patients should be immediately removed fromthe MRI room in response to any arrhythmias. Therapy is then deliveredin response to any sustained events.

FIG. 5 illustrates exemplary techniques that may be performed by thepacer/ICD in accordance with step 102 of FIG. 2. Beginning at step 218,the pacer/ICD activates an MRI transmission mode employing long range RFtelemetry using: MICS band frequencies (402 MHz-405 MHz) or ISM bandfrequencies (2.5 GHz-5.0 GHz), and any communications protocolsassociated therewith. That is, the telemetry components of the pacer/ICDare equipped to receive and transmit data at those frequencies and inaccordance with those protocols. As already noted, byreceiving/transmitting data at those frequencies, the data can becommunicated without any significant noise due to the MRI. At step 220,the pacer/ICD then begins transmitting one or more of: IEGMs;intracardiac pressure values, blood oxygen saturation values, patienttemperature values, PPG signals, tip temperature values, induced currentvalues (if calculated by the device), etc., at the MRI transmission modefrequencies. Again, these parameters are merely exemplary. A widevariety of other electrophysiological and/or hemodynamic parameters canadditionally, or alternatively, be transmitted, such as pH values, bloodglucose values, accelerometer values, stroke volume, cardiac outputvalues, contractility values, respiration, acoustic sensor values andEDV values. Assuming that the pacer/ICD initiates its MRI mode prior tothe commencement of the MRI procedure (as discussed with reference tostep 200 of FIG. 3), the pacer/ICD will be begin transmitting data priorto the application of any fields during an MRI procedure. Accordingly,the initial values that are sensed and transmitted will berepresentative of baseline values. Subsequent values sensed andtransmitted during the MRI may then be compared against the baselinevalues by the attending personnel.

At step 222, the pacer/ICD transmits warnings pertaining to any abnormalconditions detected at step 216 of FIG. 3 including: tachyarrhythmias;abnormal patient temperature; abnormal cardiac pressure; abnormal tiptemperatures and abnormal induced current levels. Any abnormalconditions detected before the initiation of the MRI procedure, such asany abnormal pressure levels or the presence of any arrhythmias, maywarrant postponement of the MRI procedure. Any abnormal condition notdetected until the MRI procedure commences may be due to the MRI fieldsand hence may warrant deactivation of the MRI machine. Any abnormalconditions persisting even after the MRI machine has been deactivatedmay warrant emergency attention, particularly any tachyarrhythmias thatare sustained well after the MRI is deactivated.

Turning now to FIGS. 6-7, operations performed by the externalmonitoring system or other external or remote system will now bedescribed in detail. In particular, FIG. 6 illustrates exemplarytechniques that may be performed in accordance with step 104 of FIG. 3.Beginning at step 300, the external monitoring system detects thepresence of the pacer/ICD based on signals received from the pacer/ICDin response to the aforementioned MRI proximity notification signalsperiodically transmitted by the external system. If no pacer/ICD isdetected within the MRI procedure room, the external system continues toperiodically transmit the notification signals, at step 302. Assuming,though, that a pacer/ICD responds to the notification signals, theexternal system then activates its long-range RF telemetry mode, at step304, to beginning receiving data from the pacer/ICD. In implementationswherein the external system is not equipped to transmit the notificationsignals or the pacer/ICD is not equipped to respond to the signals, thelong-range telemetry mode may be activated manually via appropriateinput commands provided by the attending personnel. Alternatively, thelong-range telemetry mode may remain active at all times to detect andrespond to RF telemetry signals. The latter configuration may beparticularly appropriate if the external system is a dedicatedmonitoring system provided exclusively for use in communicating withpacer/ICDs during MRI procedures. If, however, the external system is aportable programmer device that is also equipped to provide a wide rangeof pacer/ICD programming operations, then its long range MRI telemetrymode is preferably activated only when needed, i.e. only when theexternal programmer and a pacer/ICD are both in proximity to an MRImachine. In any case, as discussed above, the long range telemetry modemay exploit MICS band frequencies (402 MHz-405 MHz) or ISM bandfrequencies (2.5 GHz-5.0 GHz), and any communications protocolsassociated therewith. Whereas a particular pacer/ICD might be equippedto utilize only one of those two telemetry modes, the external system ispreferably equipped to receive and transmit in either mode, so as tocommunicate with pacer/ICDs of either type. As already explained, theuse of these communication channels helps ensure that data can bereliably transmitted from the pacer/ICD to the external system. Notchfilters and/or adaptive slew rate filters of the type described abovemay also be provided within the telemetry components of the externalsystem to filter out MRI fields and thereby provide for improvedcommunication.

At step 306, the external system then begins receiving data transmittedfrom implantable device including the aforementionedelectrophysiological signals and hemodynamic signals (e.g. IEGMs,intracardiac pressure values, blood oxygen saturation values, patienttemperature values, PPG signals, etc.), as well as tip temperaturevalues and induced current values, if provided by the pacer/ICD. Asalready noted, such signals can be received even before the MRIprocedure begins. Such initial signals are preferably stored anddisplayed as “baseline” signals for comparison against subsequentsignals received from the pacer/ICD during the actual MRI procedure. Atstep 308, the external system also receives warnings pertaining toabnormal conditions, if any, detected within the patient by thepacer/ICD. If any such warnings are received then, at step 310, theexternal system preferably forwards appropriate command signals to theMRI controller for suspending the MRI procedure. Suitable warningsignals are also presented to immediately notify the attending MRIpersonnel, such as audible or visual alarms. Also, if appropriate, thewarning signals and corresponding diagnostics data are forwarded tocardiologists or electrophysiologists at remote locations via theInternet or other communications network for further review. Typically,any such transmission of data to remote locations is performed onlyunder the control of the medical personnel in the MRI procedure room inresponse to any conditions that the medical personal cannot diagnose orare unable to address. It should be understood that the forwarding ofdata for remote review is not performed in lieu of removing a patientfrom the MRI in response to a tachyarrhythmia. Rather, any seriousarrhythmia is addressed immediately by the MRI personnel. Nevertheless,circumstances may arise where it is desirable to have IEGMs or otherdiagnostic data reviewed by experts at a remote location, timepermitting.

FIG. 7 illustrates exemplary techniques that may be performed by theexternal monitoring system in accordance with step 106 of FIG. 3. Atstep 312, the external system analyzes tip temperature signals toestimate electrical currents induced in leads by the MRI, unless thatinformation was already provided by implanted device. That is, inimplementations where the pacer/ICD is equipped to provide tiptemperature values but is not provided with the necessary software forestimating induce currents therefrom, the external system may performthat estimate (assuming it is provided with the necessary software,conversion values, etc.) Even in implementations where the pacer/ICD isequipped to provide induced current estimates, the external system maybe equipped to provide more precise estimates using, e.g., moresophisticated analysis techniques than might be accommodated within thepacer/ICD.

At step 314, the pacer/ICD analyzes received signals to detect anyabnormal conditions within the patient not already detected by thepacer/ICD including: tachyarrhythmias, abnormal patient temperatures,abnormal cardiac pressures, abnormal tip temperatures, and/or abnormalinduced current levels. In this regard, more sophisticated analysistechniques may be performed by the external system than might beaccommodated within the pacer/ICD. As one example, a pacer/ICD typicallydetects and distinguishes ventricular tachyarrhythmias merely becomparing the ventricular rate against one or more thresholds, asdiscussed above. The external system; in contrast, may perform a moresophisticated analysis of the morphology of the IEGM to detect anddistinguish various arrhythmias. See, for example, U.S. Pat. No.5,404,880 to Throne, entitled “Scatter Diagram Analysis System andMethod for Discriminating Ventricular Tachyarrhythmias.” Also, at step314, newly received data signals can be compared against any previouslyreceived baseline signals, to facilitate the detection of abnormalconditions triggered by the MRI procedure.

At step 316, the pacer/ICD then takes the following actions (if notalready performed at step 310 of FIG. 6): suspend the MRI, notify MRIpersonnel, forward warnings and diagnostics data to appropriatepersonnel at remote locations for further review. Also, at step 316, theexternal system may automatically reprogram the pacer/ICD, if needed, bysending appropriate programming signals to the pacer/ICD via long-rangetelemetry. In this regard, if the external system detects an arrhythmiathat the pacer/ICD had failed to detect, the external programmer mayreprogram the pacer/ICD accordingly so as to, e.g., switch the pacer/ICDfrom a tracking mode to a non-tracking mode.

At step 318, the external system displays one or more of: the patient'sIEGM, intracardiac pressure, blood oxygen saturation, patienttemperature, PPG signals, tip temperature values, induced currentvalues, abnormal conditions, and any warnings, whether initiallygenerated by the pacer/ICD or subsequently generated by the externalsystem. Attending personal can then review the data and take action, atstep 322, when appropriate. In this regard, the attending personnelmight identity abnormal conditions that neither the pacer/ICD nor theexternal system automatically detected. The attending personnel may thenforward the data to the remote location for expert review or may takewhatever other steps are appropriate, such as manually suspending theMRI procedure (by entering appropriate commands into the input consoleof the MRI controller) or manually reprogramming the pacer/ICD (byentering appropriate re-programming commands into the input console ofan external programmer for relaying to the pacer/ICD), etc. That is, atstep 320, the external monitoring system accepts commands from attendingpersonnel, including device reprogramming commands, in response to thedisplayed data and warning signals. In some implementations, theattending personnel may specify additional diagnostics data to begenerated and forwarded by the pacer/ICD. For example, if the pacer/ICDis programmed to initially provide only IEGM data, the attendingpersonal may reprogram the pacer/ICD during the MRI procedure to begindetecting and sending additional diagnostic information (blood pressure,SO₂, etc.) that might be of particular interest.

The techniques discussed above can be implemented in a wide variety ofimplantable medical device for use with a wide variety of externalsystems. For the sake of completeness, detailed descriptions of anexemplary pacer/ICD and an exemplary external monitoring system will nowbe provided.

Exemplary Pacer/ICD

With reference to FIGS. 8 and 9, a description of the pacer/ICD of FIG.1 will now be provided. FIG. 8 provides a simplified diagram of thepacer/ICD, which is a dual-chamber stimulation device capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation, as wellas capable of detecting and responding to MRI fields.

To provide atrial chamber pacing stimulation and sensing, pacer/ICD 10is shown in electrical communication with a heart 412 by way of a leftatrial lead 420 having an atrial tip electrode 422 and an atrial ringelectrode 423 implanted in the atrial appendage. Pacer/ICD 10 is also inelectrical communication with the heart by way of a right ventricularlead 430 having, in this embodiment, a ventricular tip electrode 432, aright ventricular ring electrode 434, a right ventricular (RV) coilelectrode 436, and a superior vena cava (SVC) coil electrode 438.Typically, the right ventricular lead 430 is transvenously inserted intothe heart so as to place the RV coil electrode 436 in the rightventricular apex, and the SVC coil electrode 438 in the superior venacava. Accordingly, the right ventricular lead is capable of receivingcardiac signals, and delivering stimulation in the form of pacing andshock therapy to the right ventricle.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, pacer/ICD 10 is coupled to a “coronary sinus”lead 424 designed for placement in the “coronary sinus region” via thecoronary sinus os. The coronary sinus lead 424 includes a distalelectrode 427 adapted for placement adjacent to the left ventricleand/or additional electrode(s) 426 adapted for placement adjacent to theleft atrium. As used herein, the phrase “coronary sinus region” refersto the vasculature of the left ventricle, including any portion of thecoronary sinus, great cardiac vein, left marginal vein, left posteriorventricular vein, middle cardiac vein, and/or small cardiac vein or anyother cardiac vein accessible by the coronary sinus. Accordingly, anexemplary coronary sinus lead 424 is designed to receive atrial andventricular cardiac signals and to deliver left ventricular pacingtherapy using at least a left ventricular tip electrode 426, left atrialpacing therapy using at least a left atrial ring electrode 427, andshocking therapy using at least a left atrial coil electrode 428. Withthis configuration, biventricular pacing can be performed. Although onlythree leads are shown in FIG. 8, it should also be understood thatadditional stimulation leads (with one or more pacing, sensing and/orshocking electrodes) may be used in order to efficiently and effectivelyprovide pacing stimulation to the left side of the heart or atrialcardioversion and/or defibrillation.

Additionally, a hemodynamic sensor 437 is shown mounted to the RV lead430 that transmits one or more hemodynamic signals, such as RVP signals,to the pacer/ICD. Numerous other sensors can be mounted to the variouspacing/sensing leads or to other leads. Also, a tip temperature sensor439 is mounted near tip electrode 432 for sensing its temperature.Similar sensors may be mounted adjacent the tip electrodes of the otherleads.

A simplified block diagram of internal components of pacer/ICD 10 isshown in FIG. 9. While a particular pacer/ICD is shown, this is forillustration purposes only, and one of skill in the art could readilyduplicate, eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation aswell as providing for the aforementioned apnea detection and therapy.

The housing 440 for pacer/ICD 10, shown schematically in FIG. 9, isoften referred to as the “can”, “case” or “case electrode” and may beprogrammably selected to act as the return electrode for all “unipolar”modes. The housing 440 may further be used as a return electrode aloneor in combination with one or more of the coil electrodes, 428, 436 and438, for shocking purposes. The housing 440 further includes a connector(not shown) having a plurality of terminals, 442, 443, 444, 446, 448,452, 454, 456 and 458 (shown schematically and, for convenience, thenames of the electrodes to which they are connected are shown next tothe terminals). As such, to achieve right atrial sensing and pacing, theconnector includes at least a right atrial tip terminal (A_(R) TIP) 442adapted for connection to the atrial tip electrode 422 and a rightatrial ring (A_(R) RING) electrode 443 adapted for connection to rightatrial ring electrode 423. To achieve left chamber sensing, pacing andshocking, the connector includes at least a left ventricular tipterminal (V_(L) TIP) 444, a left atrial ring terminal (A_(L) RING) 446,and a left atrial shocking terminal (A_(L) COIL) 448, which are adaptedfor connection to the left ventricular ring electrode 426, the leftatrial tip electrode 427, and the left atrial coil electrode 428,respectively. To support right chamber sensing, pacing and shocking, theconnector further includes a right ventricular tip terminal (V_(R) TIP)452, a right ventricular ring terminal (V_(R) RING) 454, a rightventricular shocking terminal (R_(V) COIL) 456, and an SVC shockingterminal (SVC COIL) 458, which are adapted for connection to the rightventricular tip electrode 432, right ventricular ring electrode 434, theRV coil electrode 436, and the SVC coil electrode 438, respectively. Asensor terminal 459 is provided for connection to hemodynamic sensor437. A tip temperature electrode 461 is provided for connection to tiptemperature sensor 439.

A set of adaptive slew rate filters and/or notch filters 463 areprovided along the various terminals for filtering MRI-induced noisefrom input/output signals. As described above, depending upon theimplementation, some or all of these filters may be selectivelyactivated only in response to the presence of MRI signals. Amagnetometer 465 may be provided for detecting MRI fields. AnMRI-responsive sensing controller 501, discussed below, controls theactivation of the filters in response to MRI fields detected by themagnetometer.

At the core of pacer/ICD 10 is a programmable microcontroller 460, whichcontrols the various modes of stimulation therapy. As is well known inthe art, the microcontroller 460 (also referred to herein as a controlunit) typically includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling the delivery ofstimulation therapy and may further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 460 includes the ability to process or monitor inputsignals (data) as controlled by a program code stored in a designatedblock of memory. The details of the design and operation of themicrocontroller 460 are not critical to the invention. Rather, anysuitable microcontroller 460 may be used that carries out the functionsdescribed herein. The use of microprocessor-based control circuits forperforming timing and data analysis functions are well known in the art.

As shown in FIG. 9, an atrial pulse generator 470 and a ventricularpulse generator 472 generate pacing stimulation pulses for delivery bythe right atrial lead 420, the right ventricular lead 430, and/or thecoronary sinus lead 424 via an electrode configuration switch 474. It isunderstood that in order to provide stimulation therapy in each of thefour chambers of the heart, the atrial and ventricular pulse generators,470 and 472, may include dedicated, independent pulse generators,multiplexed pulse generators or shared pulse generators. The pulsegenerators, 470 and 472, are controlled by the microcontroller 460 viaappropriate control signals, 476 and 478, respectively, to trigger orinhibit the stimulation pulses.

The microcontroller 460 further includes timing control circuitry (notseparately shown) used to control the timing of such stimulation pulses(e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction(A-A) delay, or ventricular interconduction (V-V) delay, etc.) as wellas to keep track of the timing of refractory periods, blankingintervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., which is well known in the art.Switch 474 includes a plurality of switches for connecting the desiredelectrodes to the appropriate I/O circuits, thereby providing completeelectrode programmability. Accordingly, the switch 474, in response to acontrol signal 480 from the microcontroller 460, determines the polarityof the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches (not shown)as is known in the art.

Switch 474 is configured to allow any output of the device to beselectively tri-stated, i.e. the switch includes internal componentsthat allow outputs to be generated as normal bi-state outputs (positivevs. negative) or tri-sate outputs (positive, negative, open circuit.)This will be described in more detail below with reference to FIG. 6.

Atrial sensing circuits 482 and ventricular sensing circuits 484 mayalso be selectively coupled to the right atrial lead 420, coronary sinuslead 424, and the right ventricular lead 430, through the switch 474 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 482 and 484, may include dedicated senseamplifiers, multiplexed amplifiers or shared amplifiers. The switch 474determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity. Each sensing circuit, 482 and 484, preferablyemploys one or more low power, precision amplifiers with programmablegain and/or automatic gain control, bandpass filtering, and a thresholddetection circuit, as known in the art, to selectively sense the cardiacsignal of interest. The automatic gain control enables pacer/ICD 10 todeal effectively with the difficult problem of sensing the low amplitudesignal characteristics of atrial or ventricular fibrillation. Theoutputs of the atrial and ventricular sensing circuits, 482 and 484, areconnected to the microcontroller 460 which, in turn, are able to triggeror inhibit the atrial and ventricular pulse generators, 470 and 472,respectively, in a demand fashion in response to the absence or presenceof cardiac activity in the appropriate chambers of the heart.

For arrhythmia detection, pacer/ICD 10 utilizes the atrial andventricular sensing circuits, 482 and 484, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 460 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, atrialtachycardia, atrial fibrillation, low rate VT, high rate VT, andfibrillation rate zones) and various other characteristics (e.g., suddenonset, stability, physiologic sensors, and morphology, etc.) in order todetermine the type of remedial therapy that is needed (e.g., bradycardiapacing, antitachycardia pacing, cardioversion shocks or defibrillationshocks).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 490. The data acquisition system 490 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device502. The data acquisition system 490 is coupled to the right atrial lead420, the coronary sinus lead 424, and the right ventricular lead 430through the switch 474 to sample cardiac signals across any pair ofdesired electrodes. The microcontroller 460 is further coupled to amemory 494 by a suitable data/address bus 496, wherein the programmableoperating parameters used by the microcontroller 460 are stored andmodified, as required, in order to customize the operation of pacer/ICD10 to suit the needs of a particular patient. Such operating parametersdefine, for example, pacing pulse amplitude or magnitude, pulseduration, electrode polarity, rate, sensitivity, automatic features,arrhythmia detection criteria, and the amplitude, waveshape and vectorof each shocking pulse to be delivered to the patient's heart withineach respective tier of therapy. Other pacing parameters include baserate, rest rate and circadian base rate.

Advantageously, the operating parameters of the implantable pacer/ICD 10may be non-invasively programmed into the memory 494 through a telemetrycircuit 500 in telemetric communication with an external device 502,such as a programmer, transtelephonic transceiver or a diagnostic systemanalyzer, or the external monitoring system 8 (FIG. 1). The telemetrycircuit 500 is activated by the microcontroller by a control signal 506.The telemetry circuit 500 advantageously allows IEGMs and otherelectrophysiological signals and/or hemodynamic signals, tip temperatureinformation, induce current information and status information relatingto the operation of pacer/ICD 10 (as stored in the microcontroller 460or memory 494) to be sent to the external programmer device 502 throughan established communication link 504 or to a separate externalmonitoring system via link 509. To facilitate communication with theexternal monitoring system, MICs band and/or ISM band components 467 and469 are provided within the telemetry circuit. Notch filters and/oradaptive slew rate filters may be provided within the telemetry circuit,as well.

Pacer/ICD 10 further includes an accelerometer or other physiologicsensor 508, commonly referred to as a “rate-responsive” sensor becauseit is typically used to adjust pacing stimulation rate according to theexercise state of the patient. However, the physiological sensor 508 mayfurther be used to detect changes in cardiac output, changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states) and to detect arousal fromsleep. Accordingly, the microcontroller 460 responds by adjusting thevarious pacing parameters (such as rate, AV Delay, V-V Delay, etc.) atwhich the atrial and ventricular pulse generators, 470 and 472, generatestimulation pulses. While shown as being included within pacer/ICD 10,it is to be understood that the physiologic sensor 508 may also beexternal to pacer/ICD 10, yet still be implanted within or carried bythe patient, such as sensor 437 of FIG. 8. A common type of rateresponsive sensor is an activity sensor incorporating an accelerometeror a piezoelectric crystal, which is mounted within the housing 440 ofpacer/ICD 10. Other types of physiologic sensors are also known, forexample, sensors that sense the oxygen content of blood, respirationrate and/or minute ventilation, pH of blood, ventricular gradient, etc.

The pacer/ICD additionally includes a battery 510, which providesoperating power to all of the circuits shown in FIG. 9. The battery 510may vary depending on the capabilities of pacer/ICD 10. If the systemonly provides low voltage therapy, a lithium iodine or lithium copperfluoride cell may be utilized. For pacer/ICD 10, which employs shockingtherapy, the battery 510 must be capable of operating at low currentdrains for long periods, and then be capable of providing high-currentpulses (for capacitor charging) when the patient requires a shock pulse.The battery 510 must also have a predictable discharge characteristic sothat elective replacement time can be detected. Accordingly, pacer/ICD10 is preferably capable of high voltage therapy and appropriatebatteries.

As further shown in FIG. 9, pacer/ICD 10 is shown as having an impedancemeasuring circuit 512 which is enabled by the microcontroller 460 via acontrol signal 514. Herein, thoracic impedance is primarily detected foruse in tracking thoracic respiratory oscillations. Other uses for animpedance measuring circuit include, but are not limited to, leadimpedance surveillance during the acute and chronic phases for properlead positioning or dislodgement; detecting operable electrodes andautomatically switching to an operable pair if dislodgement occurs;measuring respiration or minute ventilation; measuring thoracicimpedance for determining shock thresholds; detecting when the devicehas been implanted; measuring respiration; and detecting the opening ofheart valves, etc. The impedance measuring circuit 512 is advantageouslycoupled to the switch 474 so that any desired electrode may be used.

In the case where pacer/ICD 10 is intended to operate as an implantablecardioverter/defibrillator (ICD) device, it detects the occurrence of anarrhythmia, and automatically applies an appropriate electrical shocktherapy to the heart aimed at terminating the detected arrhythmia. Tothis end, the microcontroller 460 further controls a shocking circuit516 by way of a control signal 518. The shocking circuit 516 generatesshocking pulses of low (up to 0.5 joules), moderate (0.5-10 joules) orhigh energy (11 to 40 joules), as controlled by the microcontroller 460.Such shocking pulses are applied to the heart of the patient through atleast two shocking electrodes, and as shown in this embodiment, selectedfrom the left atrial coil electrode 428, the RV coil electrode 436,and/or the SVC coil electrode 438. The housing 440 may act as an activeelectrode in combination with the RV electrode 436, or as part of asplit electrical vector using the SVC coil electrode 438 or the leftatrial coil electrode 428 (i.e., using the RV electrode as a commonelectrode). Cardioversion shocks are generally considered to be of lowto moderate energy level (so as to minimize pain felt by the patient),and/or synchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5-40joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 460 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

Insofar as MRI-responsive operations are concerned, the microcontrollerincludes an MRI-responsive signal detection controller 501, which isoperative to control the detection of signals within the patient duringan MRI procedure, wherein the detected signals include, at least,electrophysiological signals and/or hemodynamic signals, generally inaccordance with the techniques described above in connection with FIG.3. The microcontroller includes an MRI-responsive transmissioncontroller 503, which is operative to control transmission of thedetected signals to the external monitoring system during the MRIprocedure, generally in accordance with the techniques described abovein connection with FIG. 5. Moreover, the microcontroller includes a tiptemperature-based induced current detector 505, which is operative toestimate induced current based on tip temperature values received viatip temperature terminal 461, generally in accordance with thetechniques described above in connection with FIG. 3. Still further, themicrocontroller includes a MRI response abnormal condition detector 507,which is operative to detect one or more abnormal conditions within thepatient, such as abnormal temperatures, pressures, etc., also generallyin accordance with the techniques described above.

Depending upon the implementation, the various components of themicrocontroller may be implemented as separate software modules or themodules may be combined to permit a single module to perform multiplefunctions. In addition, although shown as being components of themicrocontroller, some or all of these components may be implementedseparately from the microcontroller, using application specificintegrated circuits (ASICs) or the like.

Exemplary External Monitoring System

With reference to FIG. 10, a brief description of an exemplary remotemonitoring system 8 for use in the system of FIG. 1 will now beprovided. Remote monitoring system 8 includes a telemetry circuit 600connected to long range RF antenna 16 for communicating with a pacer/ICDor other implantable medical device. The telemetry circuit includes MICSband and ISM band components, 602 and 604, for controlling communicationwith the pacer/ICD in accordance with those protocols and at thefrequencies specified for use therewith. Notch filters 606 and adaptiveslew rate filters 608, shown in block diagram form, may be provided tofilter noise from signals received from the pacer/ICD arising due to theMRI fields. An MRI proximity notification system 610 generates theperiodic MRI proximity notification signals, discussed above, fornotifying the pacer/ICD of its proximity to the MRI and for activatingthe MRI mode therein so that the pacer/ICD can then begin to transmitelectrophysiological signals, hemodynamic signals and/or other signalsto the external monitoring system. An MRI-responsive signal receptioncontroller 612 controls the reception of signals from the pacer/ICDduring the MRI procedure, generally in accordance with the techniquesdescribed above in connection with FIG. 6. An MRI-responsive dataanalysis controller 614 controls the analysis of signals received fromthe pacer/ICD during the MRI procedure, generally in accordance with thetechniques described above in connection with FIG. 7. A tiptemperature-based induced current detector 616 may be provided toestimate induced current levels within the leads of the patient based ontip temperature values received from the pacer/ICD, if that informationis not already provided.

A display controller 618 controllers the generation of graphicaldisplays of data received from the pacer/ICD and any data generatedwithin the external system for display on a graphical display device620, such as an LCD, CRT display or the like, also generally inaccordance with techniques described above in connection with FIG. 7. AnMRI-responsive abnormal condition controller 622 further analyses thediagnostic data to detect arrhythmias, abnormal temperature, pressures,etc. within the patient, also generally in accordance with techniquesdescribed above in connection with FIG. 7. Warnings pertaining to anyabnormal conditions, either detected by the pacer/ICD or by the externalsystem or both, are generated by abnormal condition warning controller624 for display via display device 620 or via an audible warning device,also generally in accordance with techniques described above inconnection with FIG. 7. An MRI controller interface unit 626 is providedfor interfacing with the controller (block 6 of FIG. 1) of the MRImachine for sending signals to the MRI controller to suspend the MRIprocedure, if warranted due to the detection of a tachyarrhythmia orother abnormal condition within the patient, also generally inaccordance with techniques described above.

What have been described are various systems and methods forMRI-responsive operations using a pacer/ICD in conjunction with anexternal monitoring system. Principles of the invention may beexploiting using other implantable systems, externals systems or inaccordance with other techniques. Thus, while the invention has beendescribed with reference to particular exemplary embodiments,modifications can be made thereto without departing from the scope ofthe invention.

What is claimed is:
 1. A method for use with an external monitoringsystem used in conjunction with an implantable medical device forimplant within a patient, the method comprising: receiving signals vialong range telemetry during a magnetic imaging procedure, the signalsbeing sensed within the patient during the magnetic imaging procedureand including a signal representative of a temperature of an electrodeof the implanted medical device, the signals being transmitted from theimplantable medical device implanted within the patient undergoing theprocedure, the signals received at frequencies selected to reduce theinfluence of magnetic imaging fields on the transmitted signals;filtering the signals using filters within the external systemconfigured to reduce the influence of MRI fields on the signals; andprocessing the signals received via long range telemetry using theexternal monitoring system during the magnetic imaging procedure suchthat signals sensed within a patient during the magnetic imagingprocedure can also be remotely monitored during the procedure.
 2. Themethod of claim 1 wherein the magnetic imaging procedure is a magneticresonance imaging (MRI) procedure.
 3. The method of claim 2 whereinreceiving the signals at the external monitoring system during the MRIprocedure includes receiving signals transmitted by the implantabledevice at frequencies associated with medical implant communicationservices (MICS) band frequencies.
 4. The method of claim 2 whereinreceiving the signals at the external monitoring system during the MRIprocedure includes receiving signals transmitted at frequenciesassociated with industrial scientific medical (ISM) band frequencies. 5.The method of claim 2 wherein receiving signals at the externalmonitoring system comprises receiving signals transmitted by theimplantable device representative of an abnormal condition detected bythe implanted device during the MRI procedure.
 6. The method of claim 2wherein receiving signals at the external monitoring system during theMRI procedure includes receiving signals transmitted by the implanteddevice representative of an amplitude of a current induced on a lead ofthe implantable device.
 7. The method of claim 2 further comprisingreceiving and displaying an estimate of an amplitude of a currentinduced within a lead of the implantable device.
 8. The method of claim2 further comprising: transmitting notification signals warning of entryinto an MRI procedure room; receiving responsive signals from animplantable device entering the MRI procedure room; and in responsethereto, activating a long-range telemetry mode within the externalsystem wherein the reception of signals from the implanted device duringthe MRI procedure is enabled.